Kong Mesh: a dataplane token without a workload binding can claim any workload's SPIFFE identity

In Kong Mesh running in universal mode with a MeshIdentity whose SPIFFE ID path template derives from the dataplane's kuma.io/workload label, the XDS authenticator in kuma-cp validates that label only when the dataplane token is bound to a workload. Workload binding is optional, so a dataplane presenting a tags-bound token can register with kuma.io/workload set to any value and obtain another workload's SPIFFE identity.